1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus. The present invention particularly relates to a transfer and fixing device for the electrophotographic image forming apparatus in which deteriorated portions on a silicone rubber with ozone and the like is removed by grinding particularly when the silicone rubber is used as a surface layer of a primary transfer roller for printing by taking advantage of surface tension differences.
2. Description of the Related Art
As a conventional art of an electrophotographic image forming apparatus, Japanese Unexamined Patent Publication (JP-A) No. 9-114269 discloses a primary transfer roller using a tubular core metal made of aluminum as a base on which rubber is coated as a lining. A conductive film made of a polycarbonate or tetrafluoroethylene resin is wound around the roller.
A toner image is firstly formed by allowing toner to adhere on a photosensitive roller charged with an electrification device. This toner image is transferred to a primary transfer roller by pressing the photosensitive roller onto the primary roller while taking advantage of a bias potential impressed on the primary transfer roller. However, it was a problem that transfer performance such as transfer efficiency is declined since the surface layer of the primary transfer roller is degenerated by the bias potential impressed during transcription.
For solving the problem above, conductive film portions on the surface of the primary transfer roller is ground by allowing a grinding device to contact the primary transfer roller, when resistance on the surface of the primary transfer roller as measured with a resistance measuring apparatus has decreased to below a permissible limit value. Grinding is stopped when the resistance level obtained by simultaneously measuring the resistance falls within the permissible range.
When a silicone rubber is used for the surface material of the primary transfer roller, on the other hand, the rubber layer covering the surface of the roller is formed by injection molding or extrusion processing of the silicone rubber, followed by a grinding finish after forming heat bridge bonds. However, the rubber layer may be also deteriorated due to ozone denaturation of the rubber layer, or because low molecular weight components that are not bound by bridge bonds (non-bridge bond portions) may efflux as a result of insufficient polymerization in the rubber layer.
Surface tension changes accompanied by deterioration of the rubber layer material also arises a decreased printing quality. This is because the critical surface tension of the surface layer of the primary transfer roller is required to be larger than the surface tension of a belt of the photosensitive surface layer, since transfer of the toner image from the surface layer of the belt of the photosensitive surface layer to the surface layer of the primary transfer roller is controlled by the difference between the critical surface tension of the belt of the photosensitive surface layer and the critical surface tension of the surface layer of the primary transfer roller.
Adhesive force between the surface layer of the primary transfer roller and the toner image should be suppressed to be lower than the adhesive force between the printing paper and the toner image, in order to readily peel the sheet of paper from the primary transfer roller after the image has been subjected to secondary transcription and fixing on the printing paper.
The range between the upper limit and the lower limit of the critical surface tension of the surface of the primary transfer roller should be strictly restricted for the purpose above.
The service life of the roller is shortened since adhesion of fine ink particles on the surface layer of the primary transfer layer can not be avoided due to processing limitations in forming the surface of the primary transfer roller, also producing a problem yet to be solved by some simple means.
It is also an additional problem that the grinding member is plugged by grinding with the primary transfer roller when a grinding roller is used, thereby grinding force is reduced since the grinding material side is abraded. Although the circumference length of the grinding member should be elongated for prolonging the service life of the grinding member, the problem is only solved by increasing the roller diameter when the grinding roller is used. However, increasing the roller diameter is contradictory to the requirement of saving the installation space of the equipment.